Image skew adjustment for a raster output scanning (ROS) system

ABSTRACT

A procedure is provided for adjusting for image line skew caused by tolerance accumulation between a Raster Output Scanning (ROS) assembly and a photoreceptor which is being scanned. The ROS housing is adjustably mounted in relation to the photoreceptor, so that incremental adjustments can be made to cause the output scan lines emerging from the ROS housing to be moved in the process or reverse process direction vis-a-vis the photoreceptor. A test print is generated which provides measurable scan delineators to identify the amount of scan line skew created by a photoreceptor misalignment. The measured scan line skew alignment is correlated with adjustments made by a hex head screw to provide a very precise alignment. The effective adjustment range is ±4 mm in the process direction which corresponds to ±6.3 milliradians of scan line skew.

BACKGROUND OF THE INVENTION

The present invention is generally directed towards an improved mountingfor a ROS housing containing the optical components used to direct amodulated scanning beam onto the surface of a photoreceptor, and, moreparticularly, to a means and method for compensating for scan line(image) skew errors due to photoreceptor-to-ROS misalignment.

Printing systems utilizing a ROS to form images on a photoreceptorsurface are well known in the art. Conventionally, the ROS includes adiode or gas laser for generating a coherent beam of radiation; amodulator for modulating the laser output in accordance with an inputvideo image signal; and a multifaceted polygon scanner for scanning themodulated laser beam output line by line, across the surface of thephotoreceptor to form the latent image. Also included in the ROS arevarious optical components to collimate, expand, focus, and align themodulated scanning beams. These optical components are fixedly mountedwithin a housing frame, which is positioned within a printer machineframe, so that the modulated and shaped scanning beams emerging from awindow in the housing are directed in a scan line which is perpendicularto the photoreceptor surface. The lines will be formed in parallelacross the surface of the photoreceptor belt. The belt should be alignedso that these parallel lines are formed perpendicular to the directionof belt travel. It is difficult to achieve this perpendicularity,resulting in a condition referred to as "scan line skew", wherein theimage effectively takes the form of a parallelogram. This condition willaffect the images which are subsequently developed and transferred tooutput prints, the output prints exhibiting degradation of quality dueto the skew effects.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a means and methodfor an initial, and for a subsequent adjustment, where necessary, of aROS unit, so that the scan skew is practically eliminated. In thepreferred embodiment, a memory-resident, digitized test pattern isprinted which exhibits the scan line skew in fine increments.Measurements are taken which are correlated with adjustments to be madeto the ROS housing positioning to cause the housing to be pivotedtowards or away from the process direction of the photoreceptor belt. Inother words, the ROS unit is aligned so that the orientation of theoutput scan beams is correlated to whatever skew has been measured inthe photoreceptor which is being scanned. More particularly, the presentinvention is directed towards a method for compensating for scan line(image) skew in a printing machine incorporating a Raster OutputScanning (ROS) system containing a plurality of optical componentsincluding the steps of:

(a) positioning the optical components of the ROS within a housing,

(b) securing the housing to a printing machine frame at a plurality ofmounting locations including at least a first, top, inboard pivotablelocation, and a second, top, outboard mount, laterally adjustablelocation,

(c) positioning a photoreceptor member adjacent the ROS housing so thatthe ROS output beams are formed in successive, parallel scan lines alongthe width of said photoreceptor, said photoreceptor being adapted tomove in a process direction,

(e) applying a skew test pattern signal to the ROS to create a latenttest pattern image on the surface of said photoreceptor,

(f) developing said latent image and transferring said developed imageto a copy sheet,

(g) fusing said copy sheet to create a final test pattern print having aplurality of lead edge and registration edge skew delineators,

(h) calculating lead edge and registration edge skew,

(i) subtracting registration edge skew from lead edge skew to determineresultant scan line skew which represents a skewing of the photoreceptorin either the process or reverse process direction, and

(j) laterally adjusting said top outboard mount location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view depicting an electronic printing system having aprinter section which includes the ROS assembly mounted according to thepresent invention.

FIG. 2 is a block diagram depicting the major elements of the printingsystem shown in FIG. 1.

FIG. 3 is an interior view of the ROS housing of FIG. 2 showing theoptical components contained therein.

FIG. 4 is an exploded view of the ROS housing seated between walls of aprinter frame.

FIG. 5 is an enlarged perspective view of the adjustable mountingmechanism for the ROS housing of FIG. 4.

FIG. 6 is an enlarged view of the opposite side of the mechanism shownin FIG. 5.

FIG. 7 is a side view of the mechanism shown in FIG. 5.

FIG. 8 shows a test pattern output print used to determine scan lineskew adjustment.

DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, there is shown an exemplary laser basedprinting system 2, divided into a scanner section 6, controller section7, and printer section 8. The adjustable ROS housing mounting to bediscussed below is associated with printer section 8.

Referring particularly to FIGS. 2 and 3, scanner section 6 incorporatesa transparent platen 20 on which a document to be scanned is located.One or more linear arrays 24 are supported in CCD array 24 forreciprocating scanning movement below platen 20. An optical assembly(not shown) directs a narrow beam of high intensity light onto anincremental area of the platen. The reflected illumination lines fromplaten 20 and the document being scanned are focused onto array 24.Array 24, which may comprise a two row CCD photo sensor, is illuminatedby the focused band of illumination and generates image signals orpixels representative of the image scanned, which after suitableprocessing by automatic gain control circuit 32 and processor 35, areoutputted to controller section 7.

Processor 35 converts the analog image signal outputs of array 24 todigital signals and processes the digital image signals as required toenable system 2 to store and handle the image data in the form requiredto carry out the job programmed. Processor 35 also provides enhancementsand changes to the image signals such as filtering, thresholding,screening, cropping, reduction/enlarging, etc. Referring to FIG. 2,controller section 7 is, for explanation purposes, divided into an imageinput controller 40, user interface (UI) 42, system controller 44, mainmemory 46, image manipulation section 48, and image output controller50. The scanned image data input from processor 35 of scanner section 6is operated on by controller section 7. The output of controller 7operates a Raster Output Scanner (ROS) 52 in printer 8. ROS 52 consistsof a housing frame, described in detail below, which houses a laser,modulator, polygon scanner, and other optical elements required to focusand direct output beams 54, which are in turn scanned across a movingphotoreceptor located in print module 56. Image lines are exposed at thephotoreceptor with each scan to create a latent electrostatographicimage of the document being imaged. The latent image can then bedeveloped and transferred to a copy sheet with the transferred imagebeing fused. A further, more detailed description of the exemplaryprinting system 2 is found in co-pending U.S. Ser. No. 07/546,617, U.S.Pat. No. 5,164,842 assigned to the same assignee as the presentinvention, whose contents are incorporated by reference.

According to the present invention, and referring to FIG. 3, ROS unit 52consists of a modular housing 60, within which are mounted the opticalcomponents which generate the scan lines at the surface of aphotoreceptor belt 62, housed in print module 56. As shown in FIG. 3,these components include a laser 64 which generates a collimated beam ofmonochromatic light. The monochromatic light beam is reflected by firstmirror 66 onto modulator 68, the light beam being modulated inconformance with the information contained in the video signal sent fromimage output control 50. Modulator 68 may be any suitable acousto-opticor electro-optical modulator for recording the video information in theform of a modulated light beam, at the output of the modulator. By meansof the modulator, the information within the video signal is representedby the modulated light beam 70. Light beam 70 is reflected by secondmirror 72 and is incident on imaging lens 74. Imaging lens 74 produces abeam 76, which is reflected by third and fourth mirrors 78 and 80, andimpinges upon a plurality of contiguous facets of scanning polygon 82.The beams reflected from polygon 82 are directed through post-polygonconditioning optics 84, and then through window 86 to form successive,parallel output raster lines at belt 62, moving in the direction ofarrow 85.

As described above, if the photoreceptor belt is skewed in the processdirection, the scanned line will not be perpendicular to the belt edgesor parallel to the lead edge of the images in the process direction. Theresulting output prints will exhibit copy quality defects resulting fromthe scan line skew.

According to the present invention, the ROS housing 60 is mounted withinprinter section 8 in such a way as to enable adjustment of the housingcomponents to compensate for the housing-to-belt skew.

ROS housing 60 is mounted within printer section 8 by a three pointmounting arrangement, best shown in FIGS. 4 and 5. FIG. 4 shows outboardand inboard machine frames 90, 92 forming the front and rear walls ofprinter section 8. Circled areas 94, 96, 98 identify those areas offrames 90, 92 which interface with and support the ROS housing 60.Considering first area 96, an inboard block 100 is fixedly attached tothe top of frame 92 by screw 102. A ball stud 104, connected to theinboard end of the ROS housing, seats within conically shaped groove 106and is held in place by clamp 108. Clamp 108 is movable laterally andengages the side of stud 104 to maintain it in its fixed, seatedposition. Clamp 108 is secured to the block 100 by screw 110.

Considering next the circled area 98, the bottom right corner of housing60 is designed to be engaged between a lateral spring 112 and a facingsurface 92A.

Turning finally to circled area 94, shown in FIG. 4 and in two enlargedviews in FIGS. 5 and 6, a second, outboard block 114 is slidably mountedon a rail 116, formed at the top of frame 90. A V-shaped notch 118 isformed in block 114, which seats a second, outboard ball stud 120,attached to the outboard side of the housing. Clamp 122 is movablelaterally and engages the side of stud 120 to maintain it in its seatedposition within notch 118. Clamp 122 is secured to block 114 by screw124.

Summarizing the above description, the ROS housing 60 is mountedvertically between frames 90, 92, with the weight of the housing beingborne at interface areas 94, 96. Lateral movement of the bottom half ofthe ROS housing is restrained by spring 112 acting against machine frame92A at area 98. This three point mounting provides the verticalROS-to-belt spacing within required tolerance. However, each beltarchitecture used in each printer 2 will be slightly different with thepotential for belt misalignment in the skew (process) direction.Provision is made for allowing scan line skew adjustment by moving block114 along guide rail 116, while allowing ball stud 104 to pivot withingroove 106. The mechanism for accomplishing this is shown in theenlarged view of FIGS. 5, 6, and 7. As best shown in FIGS. 6 and 7, oneend of block 114 terminates in two L-shaped jaw like members, 126, 128,forming a T-shaped aperture 130 within the block. An adjustment screw132 has a threaded portion 132A, which is threadingly engaged throughthreaded plate 134, fixedly attached to rail 116. The screw has asecond, non-threaded section 132B which is held in contact between jawmembers 126, 128. The end of the screw 132 terminates in a lug 136. Thehead of screw 132 is formed with a number of hex flats, 137, 137A, 137N,etc., for purposes described below. It will be apparent that, as thescrew is turned, for example, clockwise in FIG. 7, threaded portion 132Awill exert a lateral force on block 114, moving it to the right.Counterclockwise rotation of screw 132 will cause lug 136 to engage theinner surface of jaw members 126, 128, sliding block 114 to the left.This adjustment mechanism enables a very precise positioning of the ROShousing to compensate for belt skew misalignment, as will be seen in theexamples below.

ADJUSTMENT EXAMPLE

It is assumed that a ROS 52 and a print module 56 have been initiallypositioned within printer section 8 (FIG. 1). A ROS housing 60 has beenplaced into an initial seated position, which is outside tolerances setfor image scan line skew with photoreceptor 62. Controller 7 is modifiedto allow an operator tech rep to generate a skew test pattern printsignal from interface 42. Video signals from image adjust output controlcircuit 50 drive the ROS 52 to form a plurality of test pattern imageson the photoreceptor, which are then printed out as successive copies,using the xerographic processes described in the previously referencedapplication 07/546,617. A typical test pattern print 140 is shown inFIG. 8. As shown, there are a pair of register edge skew delineators140, 142 with incremental marks 140-1 through 140-10 and 142-1 through142-10. there are also a pair of lead edge skew delineators 144, 146,with incremental marks 144-1 through 144-10 and 146-1 through 146-10.Using a 150 mm scale graduated to 0.5 mm and an eye loop, the print ismeasured for registration and lead edge skew to the nearest 0.1 mm. Forexample, two registration edge measurements are made, for example,between marks 140-8 and 142-8 to the register edge 150 of print 140. Thedifference between the two measurements is the registration errorintroduced by the paper feed path of the test pattern print. Lead edgeskew measurements are then made from, for example, marks 144-8 and 146-8to lead edge 152 of print 140. The difference between these twomeasurements is the lead edge skew. The registration edge skew (if any)is subtracted from this value to arrive at the scan line skew. Thisvalue is an indication that the belt 62 is skewed, so that the lead edgeof an image frame, as represented by print 140, will be skewed in eitherthe process (+) direction or in a direction opposite to (-) the processdirection. An appropriate adjustment must then be made to the ROShousing to move the output beam 54 to compensate for the scan line skew.Inboard clamp 108 has been loosened, allowing ball stud 104 to pivotwithin groove 106. Outboard clamp 122 has been removed and the screws inblock 114 have been loosened. This adjustment is made by turningadjustment screw 132 in the clockwise (+) or counterclockwise (-)direction. The adjustment could be made by an iterative process, withsubsequent prints being made and examined to determine whether scan lineskew has been eliminated. However, according to another feature of thepresent invention, a correlation has been made between the rotation ofhex flat increments 137A, 137B, 137N of screw 132 and the desired skewmeasurement.

The following Adjustment Table is based on measurements taken from thetest pattern document over a span of 200 mm. One increment of theadjustment screw hex flat equates to a 0.082 mm correction over the 200mm distance. The number in the left column is the calculated scan lineskew derived from the above procedure. The number in the correspondingright column identifies the number of hex flats on adjustment screw 132to increment (CW or CCW), to bring the scan line skew contribution tolead edge skew to approximately zero.

    ______________________________________                                        ADJUSTMENT TABLE                                                                            HEX FLAT                                                        SCAN LINE     INCREMENTS                                                      SKEW (mm)     CW/CCW                                                          ______________________________________                                        0.08           1                                                              0.16           2                                                              0.25           3                                                              0.33           4                                                              0.41           5                                                              0.50           6                                                              0.57           7                                                              0.66           8                                                              0.74           9                                                              0.82          10                                                              0.90          11                                                              0.98          12                                                              1.07          13                                                              1.15          14                                                              1.23          15                                                              1.31          16                                                              1.39          17                                                              1.48          18                                                              1.56          19                                                              1.64          20                                                              1.72          21                                                              1.80          22                                                              1.89          23                                                              1.97          24                                                              2.05          25                                                              ______________________________________                                    

Assume, for example, that the difference between the registration edgemeasurements taken at registration edge delineators 140 and 142 is(-0.25 mm) and that the difference between the measurements taken at thelead edge delineators 144, 146 is zero. For this case, the scan lineskew=(lead edge skew-reg. edge skew)=(0-(-0.25)=+0.25 mm. For thisvalue, the ROS is pivoted in the CW direction with respect to the P/R.From the table, screw 132 is rotated through 3 hex flat increments. Asshown in FIGS. 4 and 5, clockwise rotation moves the block 114 to theright, causing housing 60 to pivot about inboard interface 96. When the3 flat increments have been made, the block is resecured to the guiderail, clamp 122 is replaced, and inboard clamp 108 is retightened.

The above example assumed an initial installation of a ROS tophotoreceptor. However, the procedures are equally applicable toadjustments which may be required after initial alignment, and aftersome period of operation during which the photoreceptor experiencesmisalignment due to various mechanical wear factors, or partsreplacement, or if a ROS is replaced in the field.

While the invention has been described with reference to the structuredisclosed, it will be appreciated that numerous changes andmodifications are likely to occur to those skilled in the art, and it isintended to cover all changes and modifications which fall within thetrue spirit and scope of the invention.

What is claimed is:
 1. A method for compensating for scan line (image)skew in a printing machine incorporating a Raster Output Scanning (ROS)system containing a plurality of optical components including the stepsof:(a) positioning the optical plurality of components of the ROS,within a housing, (b) securing the housing to a printing machine frameat a plurality of mounting locations including at least a first, top,inboard pivotable location, and a second, top, outboard mount laterallyadjustable location, (c) positioning a photoreceptor member adjacent theoutput beams generated by said ROS housing so that the ROS output beamsare formed in successive, parallel scan lines on said photoreceptor,said photoreceptor moving in a process direction, (e) applying a skewtest pattern signal to the ROS to create a latent test pattern image onsaid photoreceptor, (f) developing said latent test pattern image toform a developed image and transferring said developed image to a copysheet, (g) fusing said copy sheet to create a final test pattern printhaving a plurality of lead edge and registration edge skew delineators,(h) calculating lead edge and registration edge skew, (i) subtractingregistration edge skew from lead edge skew to determine resultant scanline skew which represents a skewing of the photoreceptor in either theprocess or reverse process direction, and (j) laterally adjusting saidtop outboard mount location.
 2. In a printing system including a RasterOutput Scanner (ROS) comprising a plurality of optical components whichscans a charged surface of a photoreceptor moving in a process directionin response to input video data signals to form image scan lines on saidphotoreceptor, and wherein the optical components comprising said ROSare contained within a generally rectangular optical housing which isvertically mounted in a printer frame in alignment with saidphotoreceptor, so as to couple output scan beams from the ROS onto thecharged photoreceptor surface, said housing having an inboard end and anoutboard end, the improvement wherein the inboard end of the ROS housingis pivotably mounted to said printer frame, with the outboard end beingadjustably mounted to said printer frame, so as to enable the ROShousing to be pivoted with respect to said process direction, wherebyany scan line skew caused by ROS to photoreceptor misalignment iscompensated for.
 3. A mounting mechanism for adjusting skew of outputscan lines formed on a surface of a photoreceptor moving in a processdirection by a ROS, said ROS comprising a plurality of opticalcomponents secured within a generally rectangular modular housing, themechanism including:(a) a machine frame providing a first, second, andthird mounting interface for the housing, said first and secondinterfaces supporting the housing, and wherein the third interfacemaintains a vertical orientation of the housing, said first interfacefurther permitting a pivoting rotation of the housing thereabout, saidsecond interface including a skew adjustment mechanism for moving oneend of the housing in a process or reverse process direction inconjunction with said pivoting rotation, and (b) means for determiningan amount of adjustment required to adjust for said skew.
 4. Themechanism of claim 3 wherein said skew adjustment mechanism includes ablock member adapted to seat the outboard end interface of the housing,said block member slidably movable on a surface of a guide rail formedon said machine frame, and a horizontally mounted screw operativelyconnected to said block member, so as to move said block member andhence, said ROS housing in a first or second lateral direction,depending upon a given screw rotation direction, said first or secondlateral direction corresponding to the process or to a non-processdirection, respectively.
 5. The mechanism of claim 3 wherein said firstand second interfaces each include a ball stud, seated within a matingdepression, formed within said block member.
 6. The mechanism of claim 4wherein a head of said horizontally mounted screw has a plurality of hexflats, each hex flat associated with a specific scan line skewadjustment.